1. Field of the Invention
The present invention relates to a data storage apparatus in which the movement of a head over a data storage medium is servoed on the basis of servo patterns.
Recently, a high-speed data processing in data processing units is demanded. For example, a high-speed data reading and writing in a magnetic disk apparatus is demanded. Therefore, it is desired that accurate information be obtained relating to the position of the magnetic head as the head moves over the disk, the seek operation be stabilized, and the settling time required to reach a predetermined track be reduced.
2. Description of the Prior Art
Conventional methods of determining the position of a magnetic head of a magnetic disk apparatus include a method whereby position determination is performed such that a magnetic head position signal is created on the basis of a servo signal obtained by reading servo patterns recorded on the magnetic disk. Since the sector servo method and the servo surface servo method are similar to each other, the sector servo method is taken as an example in the following description.
FIGS. 1A and 1B explain the servo control based on conventional two-phase servo patterns. FIG. 1A is a schematic diagram showing a locus of the head as it moves over the magnetic disk; FIG. 1B is a waveform diagram of a reproduction output of the head; and FIG. 2 explains a method of producing a position signal.
FIG. 1A shows a part of a servo area 12 formed at the head of a sector, the sector being obtained by dividing a magnetic disk 11 radially into a predetermined number of areas. On each of concentrically formed predetermined number of tracks, an AGC (auto gain controller) pattern and four patterns N+, N-, Q+ and Q- are recorded.
Positive servo patterns N+, Q+ and negative servo patterns N-, Q- are provided so that two-phase servo information N and two-phase servo information Q can be detected and processed at a high precision by means of a differential circuit. There is a phase shift of 1/4 of a cycle provided between one pattern and the next disposed on each track. A magnetic head 13 whose position is sought reads the four patterns N+, N-, Q+ and Q- so that the two-phase servo information N and the two-phase servo information Q are obtained.
The two-phase servo information N and the two-phase servo information Q are obtained, as per the following formulas, as a difference between amplitudes of signals obtained as a result of reading positive and negative servo patterns. EQU N=(N+)-(N-) (1) EQU Q=(Q+)-(Q-) (2)
As shown in FIG. 2, the smaller, in magnitude, of the information N and the information Q is employed as position information. The smaller one must be employed because there is a need to select that part of a sinusoidal position signal which exhibit a good linearity.
Conventionally, the four servo patterns N+, N-, Q+ and Q- are recorded such that two kinds of sequences (N+, Q+, N-, Q-) and (N+, N-, Q+, Q-) are repeated. The position of the magnetic head 13 is determined at each sampling on the basis of the aforementioned position information so as to determine the velocity at which the magnetic head 13 is to be moved and thus to control the moving velocity, the determination of the velocity being performed on the basis of a difference between the current position and the position obtained at the previous sampling.
The position of the magnetic head 13 is determined such that N and Q are obtained by sampling, a rough determination on a position is made on the basis of the comparison of N and Q in absolute values, and a strict determination is made on the basis of one of the four values .+-.N and .+-.Q, the one value being selected on the basis of the result of comparison between N and Q. Sampling timing is set such that one sampling operation is followed by another before the head moves across two tracks, since the position signals N and Q vary at a cycle of two tracks.
A detailed description will now be given, by returning to FIGS. 1A and 1B. The magnetic head 13 reproduces an AGC pattern, whereby a reproduction gain is controlled such that the reproduction output has a predetermined level. When the reproduction is completed, the reproduction gain is fixed. The servo patterns N+, Q+, N- and Q- are reproduced in a state in which the AGC is not activated. The AGC pattern serves as a reference against which the timing for holding the reproduction output of the servo patterns N+, Q+, N-, Q- at their peaks is produced.
Assuming that the reproduction output obtained when the entirety of the magnetic head 13 passes over a servo pattern is 1, the reproduction output obtained when 40% of the magnetic head 13 passes over the servo pattern N+ at a position P.sub.1 is 0.4. Subsequently, at a position P.sub.2, the magnetic head 13 does not passes over the servo pattern Q+ so that the reproduction output is 0. At a position P.sub.3, 40% of the magnetic head 13 passes over the servo pattern N-, and the reproduction output is 0.4. At a position P.sub.4, 90% of the magnetic head passes over the servo pattern Q-, and the reproduction output is 0.9.
As a result, the position signal N is 0 (0.4-0.4=0), and the position signal Q is -0.9 (0-0.9). Therefore, if the magnetic head 13 is tracking a track m, the position signal N is 0, and the position signal Q is 0.9.
The magnetic head 13 is in movement, and the position signal N is produced on the basis of the reproduction outputs obtained at the positions P.sub.1 and P.sub.3. Therefore, a reproduction signal providing an appearance that the magnetic head 13 is located at the position P.sub.2 between the position P.sub.1 and the position P.sub.3. Similarly, the position signal Q is produced on the basis of the reproduction output obtained at the positions P.sub.2 and P.sub.4. Therefore, a reproduction signal providing an appearance that the magnetic head 13 is located at the position P.sub.3 between the position P.sub.2 and the position P.sub.4.
FIG. 2 shows a method of producing a positional signal. As shown in FIG. 2, P.sub.2 indicates a position at which the position signal N is 0, and P.sub.3 indicates a position at which the position signal Q is -0.9. Accordingly, the position of the magnetic head 13 is determined to be the position P.sub.2 on the basis of the position signal N. Assuming that P.sub.12 indicates a position at which the next sampled value of N is obtained, and that P.sub.13 indicates a position at which the next sampled value of Q is obtained, as shown in FIG. 2, the position of the magnetic head 13 is obtained on the basis of the position signal Q as a result of comparing the positional signal N and the positional signal Q.
Thus, the distance over which the magnetic head has traveled is determined to be the distance between the position P.sub.2 and the position P.sub.13.
However, since the servo patterns are arranged in the aforementioned order, the positions indicated by the position information N and by the position information Q differ from each other. Servo control is performed at predetermined intervals by sampling servo technology. An accurate position of the magnetic head 13 can not be determined when the position indicated by the position information N and that indicated by the position information Q differ from each other.
The distance that the magnetic head 13 travels is actually the distance between the position P.sub.2 and the position P.sub.12 (or the distance between the position P.sub.3 and the position P.sub.13) when the distance between the position P.sub.2 and the position P.sub.13 is assumed.
Accordingly, an erroneous calculation of a velocity may be made during a velocity control because erroneous distance data is referred to. Especially, an erroneous calculation of a velocity may be made when the magnetic head is slowed down before reaching a target track. The magnetic head 13 may be overshot or undershot with respect to the target track. This causes a read operation or a write operation to be delayed. In some cases, the magnetic head has to wait for one full rotation of the disk.
The above problem becomes noticeable when an acceleration and a velocity in a seek mode are increased for shorter access time. When the problem occurs, the seek operation becomes unstable, and the settling time required to reach a desired track becomes long, thus making it impossible to speed up the operation.